A multiwavelength study of the Magellanic-type galaxy NGC 4449-I. Modelling the spectral energy distribution, the ionization structure and the star formation history
Résumé
We present an integrated photometric spectral energy distribution (SED) of the Magellanic-type galaxy NGC 4449 from the far-ultraviolet (UV) to the submillimetre, including new observations acquired by the Herschel Space Observatory. We include integrated UV photometry from the Swift Ultraviolet and Optical Telescope using a measurement technique which is appropriate for extended sources with coincidence loss. In this paper, we examine the available multiwavelength data to infer a range of ages, metallicities and star formation rates for the underlying stellar populations, as well as the composition and the total mass of dust in NGC 4449. Our analysis of the global optical spectrum of NGC 4449 fitted using the spectral fitting code STARLIGHT suggests that the majority of stellar mass resides in old (greater than or similar to 1 Gyr old) and metal-poor (Z/Z(circle dot) similar to 0.2) populations, with the first onset of star formation activity deduced to have taken place at an early epoch, approximately 12 Gyr ago. A simple chemical evolution model, suitable for a galaxy continuously forming stars, suggests a ratio of carbon to silicate dust mass comparable to that of the Large Magellanic Cloud over the inferred time-scales. We present an iterative scheme, which allows us to build an in-depth and multicomponent representation of NGC 4449 `bottom-up', taking advantage of the broad capabilities of the photoionization and radiative transfer code MOCASSIN (MOnte CArlo SimulationS of Ionized Nebulae). We fit the observed SED, the global ionization structure and the emission line intensities, and infer a recent star formation rate of 0.4 M-circle dot yr(-1) and a total stellar mass of approximate to 1 x 10(9) M-circle dot emitting with a bolometric luminosity of 5.7 x 10(9) L-circle dot. Our fits yield a total dust mass of 2.9 +/- 0.5 x 10(6) M-circle dot including 2 per cent attributed to polycyclic aromatic hydrocarbons. We deduce a dust to gas mass ratio of 1/190 within the modelled region. While we do not consider possible additional contributions from even colder dust, we note that including the extended H I envelope and the molecular gas is likely to bring the ratio down to as low as similar to 1/800.
Domaines
Astrophysique [astro-ph]
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